1. Field of the Invention
The present invention generally relates to preparing samples for characterization, and more particularly to preparing samples for TEM (Transmission Electron Microscopy) from supported catalysts containing nanometer sized metal particles.
2. Description of the Related Art
As the size of metal particles used in catalysts has decreased to the nanometer level (roughly 1 to 100 nm), it has become increasingly difficult to characterize the physical properties of such metal particles. TEM is a common technique used to obtain 2-D images of nanometer sized particles, and is capable of resolving particle sizes down to the sub-nanometer level. An article by Zhong L. Wang, Janet M. Petroski, Travis C. Green, and Mostafa A. El-Sayed entitled, “Shape Transformation and Surface Melting of Cubic and Tetrahedral Platinum Nanocrystals,” Journal of Physical Chemistry B, Vol. 102, pp. 6145=6151 (1998), shows a traditional TEM sample preparation method. Although this technique works fine for particles with size larger than 5 nm, in practice, it has been proved to be difficult to clearly resolve tiny particles (<2 nm) on supports, because of the noisy image background caused by the supports. As a result, physical properties of these particles are often inaccurately characterized.
Another common technique that is used in the prior art for preparing characterization samples of supported catalysts for TEM was mentioned in an article by Jung Whan Yoo, David Hathcock, and Mostafa A. El-Sayed entitled, “Characterization of Pt Nanoparticles Encapsulated in Al2O3 and Their Catalytic Efficiency in Propene Hydrogenation,” Journal of Physical Chemistry A, Vol. 106, pp. 2049-2054 (2002), and is illustrated in FIG. 1. In Step 11, the supported catalyst is ground or milled into fine powder. Then, the supported catalyst in its powder form is mixed into an organic solvent, such as acetone or ethanol (Step 12). In Step 13, an etchant is added to the solvent to digest the supports and thereby release the metal particles. The resulting solution is then placed in a centrifuge to sediment the metal particles, and then the top liquid etchant is removed. (Steps 14 and 15). Thereafter, the sediment metals are rinsed by water and centrifuged several more times (Step 16). Subsequently, a sample is extracted from the solution (Step 17) and applied to a copper grid that is used by the TEM (Step 18). Before the copper grid is placed in the TEM for imaging, the sample is dried (Step 19).
FIG. 2 is an image obtained by the TEM from the characterization sample prepared from an alumina-supported platinum catalyst according to the process of FIG. 1. The scale for this image is shown on the image at the bottom left hand corner as 10 nm. All the particles in the image represent platinum. The different grades of shade are due to particle orientation and size. Because of the agglomeration of particles, boundaries are difficult to draw. The area data is compiled for all particles whose boundaries can be resolved in the TEM image, and the diameters of the particles are derived from the area data using conventional techniques. Usually, several TEM images are taken from one sample, and the total number of analyzed particles is counted.
FIG. 3 is a graph showing the diameter distribution of platinum particles in a characterization sample prepared from an alumina-supported platinum catalyst in accordance with the process of FIG. 1. The total number of particles was determined to be 310. The diameter range of the platinum particles was determined to be 3.8-20.6 nm and the average diameter of the platinum particles was determined to be 11.3 nm.
The accuracy of the sample prepared according to FIG. 1 could not be verified when the same alumina-supported platinum catalyst was characterized using a chemisorption (CO absorption) method, which provides for a fairly accurate measure of the average diameter. Using the chemisorption method, the average diameter of the platinum particles was determined to be 2.8 nm, more than 8 nm smaller than the average diameter determined from the sample prepared according to FIG. 1. It is obvious that sample preparation following the process of FIG. 1 has the following drawbacks: a) small particles may not sediment during centrifuge, and they may be lost in the rinse process; and b) particles agglomerate together. As a result, small particles may be overlooked during image analysis.